Idle RPM climbs at stoplight.


XS400 Member
Reaction score
Council Bluffs, Iowa
I have a 1978 XS400E all original. The problem is that when the engine is hot, it will not idle down unless i use the brake. I can be cruising along and take my hand off the throttle and it will stay at speed until I brake. Then the engine will slow down. And often waiting at a stoplight it will idle fine for 6 seconds then RPM will climb right up, like 3000rpm. I hit the kill switch and start it again when the light changes.

I have adjusted the idle mixture screw out, and now I am about 3 turns from seated. I think that is a half turn too far. It's starting to bog down from that. That doesn't effect the RPM climbing thing.

I suspect it has something to do with the secondary jet and that vacuum diaphragm jet that runs it. I don't understand how it works. Could the spring be too weak? Is the diaphragm have a vent to atmosphere plugged?

Most likely causes:

Sticky vacuum piston
Hole in the diaphragm
Air leak somewhere in the intake
Stiffened and leaking throttle shaft seals

I had this with shaft seals - when hot, the hardened rubber would change shape and how much air it let in.
All good points, Thanks.
I did look through the carburetor and got everything moving correctly. I found the problem was the carb synchronization.
First I put a vacuum gauge on the right carb and found it was bouncing around zero. When I put it on the left carb it was about 30.
I didn't have a tool to compare the vacuum on each carb. So with an idea from this website I made one.
This was the problem. One carb was rich and the other was lean.
I drove it to work this morning and it is fixed. It is still a little lean, but that is an easy fix.

IMG_20220723_074807_resized.jpg IMG_20220723_074449_resized.jpg